Electronics Quiz | Basic Electronics Engineering MCQ | Advanced Level Practice Test
1 >>A d.c. shunt machine has an armature resistance of 0.5 Ω and a field-circuit resistance of 750 Ω. When run under test as a motor, with no mechanical load, and with 500 V applied to the terminals, the line current was 3 A. Allowing for a drop of 2 V at the brushes, estimate the efficiency of the machine when it operates as a generator with an output of 20 kW at 500 V, the field-circuit resistance remaining unchanged. State the assumptions made. ?
(A) 4.61 kW, 0.835 p.u.
(B) 0.893 p.u.
(C) 7.13 kW, 0.886 p.u.; 9.01 kW, 0.893 p.u.
(D) 1.008 A
2 >>in a certain transformer, the hysteresis loss is 300 W when the maximum flux density is 0.9 T and the frequency 50 Hz. What would be the hysteresis loss if the maximum flux density were increased to 1.1 T and the frequency reduced to 40 Hz? Assume the hysteresis loss over this range to be proportional to (Bmax)1.7. ?
(A) 1.11 J
(B) 337 W
(C) 800 W
(D) 6.49 kW, 0.843 p.u.
3 >>The hysteresis loop for a certain ferromagnetic ring is drawn to the following scales: 1 cm to 300 A and 1 cm to 100 ?Wb. The area of the loop is 37 cm2. Calculate the hysteresis loss per cycle. ?
(A) 1.11 J
(B) 337 W
(C) 800 W
(D) 6.49 kW, 0.843 p.u.
4 >>A d.c. series motor, having armature and field resistances of 0.06 Ω and 0.04 Ω respectively, was tested by driving it at 200 r/min and measuring the open-circuit voltage across the armature terminals, the field being supplied from a separate source. One of the readings taken was: field current, 350 A; armature p.d., 1560 V. From the above information, obtain a point on the speed/current characteristic, and one on the torque/ current characteristic for normal operation at 750 Vand at 350 A. Take the torque due to rotational loss as 50 N m. Assume that brush drop and field weakening due to armature reaction can be neglected. ?
(A) 595 r/min
(B) 40 A, 695 r/min
(C) 333 N m
(D) 917 r/min, 2560 N m
5 >> A d.c. series motor connected across a 460 V supply runs at 500 r/min when the current is 40 A. The total resistance of the armature and field circuits is 0.6 Ω.Calculate the torque on the armature in N m. ?
(A) 595 r/min
(B) 40 A, 695 r/min
(C) 333 N m
(D) 917 r/min, 2560 N m
Download
6 >> A shunt motor, connected across a 440 V supply, takes an armature current of 20 A and runs at 500 r/min.The armature circuit has a resistance of 0.6 Ω. If the magnetic flux is reduced by 30 per cent and the torque developed by the armature increases by 40 per cent,what are the values of the armature current and of the speed? ?
(A) 595 r/min
(B) 40 A, 695 r/min
(C) 333 N m
(D) 917 r/min, 2560 N m
7 >>A d.c. shunt motor runs at 900 r/min from a 480 V supply when taking an armature current of 25 A. Calculate the speed at which it will run from a 240 V supply when taking an armature current of 15 A. The resistance of the armature circuit is 0.8 Ω. Assume the flux per pole at 240 V to have decreased to 75 per cent of its value at 480 V. ?
(A) 595 r/min
(B) 40 A, 695 r/min
(C) 333 N m
(D) 917 r/min, 2560 N m
8 >>A six-pole, lap-wound, 220 V, shunt-excited d.c. machine takes an armature current of 2.5 A when unloaded at 950 r/min. When loaded, it takes an armature current of 54 A from the supply and runs at 950 r/min. The resistance of the armature circuit is 0.18 Ω and there are 1044 armature conductors. For the loaded condition, calculate: (a) the generated e.m.f.; (b) the useful flux per pole; (c) the useful torque developed by the machine in N m. ?
(A) 210.3 V, 0.0127 Wb, 114.3 N m
(B) 356 N m
(C) 5.8 Ù, 60 V
(D) 35 Ù, 962 r/min
9 >>Calculate the torque, in N m, developed by a d.c.motor having an armature resistance of 0.25 Ω and running at 750 r/min when taking an armature current of 60 A from a 480 V supply. ?
(A) 210.3 V, 0.0127 Wb, 114.3 N m
(B) 356 N m
(C) 5.8 Ù, 60 V
(D) 35 Ù, 962 r/min
10 >>Explain why a d.c. shunt-wound motor needs a starter on constant-voltage mains. A shunt-wound motor has a field resistance of 350 Ω and an armature resistance of 0.2 Ω and runs off a 250 V supply. The armature current is 55 A and the motor speed is 1000 r/min. Assuming a straight-line magnetization curve, calculate: (a) the additional resistance required in the field circuit to increase the speed to 1100 r/min for the same armature current; (b) the speed with the original field current and an armature current of 100 A. ?
11 >>A shunt machine is running as a motor off a 500 V system, taking an armature current of 50 A. If the field current is suddenly increased so as to increase the flux by 20 per cent, calculate the value of the current that would momentarily be fed back into the mains. Neglect the shunt current and assume the resistance of the armature circuit to be 0.5 Ω. ?
(A) 208 V, 41 A
(B) 140 A
(C) 468 V, 1075 r/min, 333 N m
(D) 190 V
Download
12 >>Explain clearly the effect of the back e.m.f. of a shunt motor. What precautions must be taken when starting a shunt motor? A four-pole d.c. motor is connected to a 500 V d.c.supply and takes an armature current of 80 A. The resistance of the armature circuit is 0.4 Ω. The armature is wave-wound with 522 conductors and the useful flux per pole is 0.025 Wb. Calculate: (a) the backe.m.f. of the motor; (b) the speed of the motor; (c) the torque in N m developed by the armature ?
(A) 208 V, 41 A
(B) 140 A
(C) 468 V, 1075 r/min, 333 N m
(D) 190 V
13 >>Derive the e.m.f. equation for a d.c. machine.A d.c. shunt motor has an armature resistance of 0.5 Ω and is connected to a 200 V supply. If the armature current taken by the motor is 20 A, what is the e.m.f. generated by the armature? What is the effect of: (a) inserting a resistor in the field circuit; (b) inserting a resistor in the armature circuit if the armature current is maintained at 20 A? ?
(A) 208 V, 41 A
(B) 140 A
(C) 468 V, 1075 r/min, 333 N m
(D) 190 V
14 >>Calculate the number of turns per pole required for the commutating poles of the d.c. generator referred to in Q . 11, assuming the compole ampere-turns per pole to be about 1.3 times the armature ampere-turns per pole and the brushes to be in the geometric neutral. ?
(A) 8
(B) 4440 At, 555 At; 0.463 A
(C) 200 At, 3400 At
(D) 0.0333 Wb
15 >>Draw a labelled diagram of the cross-section of a four-pole d.c. shunt-connected generator. What are the essential functions of the field coils, armature, commutator and brushes? The e.m.f. generated by a four-pole d.c. generator is 400 V when the armature is driven at 1000 r/min. Calculate the flux per pole if the wave-wound armature has 39 slots with 16 conductors per slot. ?
(A) 0.0192 Wb
(B) 140 V
(C) 1065 r/min
(D) 0.0274 Wb
16 >>The input power to a turbine in a tidal stream is ρAv3. For a turbine efficiency of 25 per cent, having a diameter of 8 m, what is the power that can be extracted from a stream of flow velocity 5 m/s? (The density of sea water ρ = 1020 kg/m3.) ?
(A) 802 kW
(B) 24 GW h, 75 686 GW h, 22.4%
(C) 1.74 × 106 kW h
(D) 3.6 × 105 m3
17 >>Explain how a rotating magnetic field may be produced by stationary coils carrying three-phase currents. Determine the efficiency and the output kilowatts of a three-phase, 400 V induction motor running on load with a fractional slip of 0.04 and taking a current of 50 A at a power factor of 0.86. When running light at 400 V, the motor has an input current of 15 A and the power taken is 2000 W, of which 650 W represent the friction, windage and rotor core loss. The resistance per phase of the stator winding (deltaconnected) is 0.5 ?. ?
(A) 1:0.333:0.16; 1:0.333:0.16
(B) 0.857 p.u., 25.57 kW
(C) 11 kW, 444 W
(D) 27.5 per cent, 41 per cent
18 >>Describe, in general terms, the principle of operation of a three-phase induction motor.The stator winding of a three-phase, eight-pole,50 Hz induction motor has 720 conductors, accommodated in 72 slots. Calculate the flux per pole of the rotating field in the airgap of the motor, needed to generate 230 V in each phase of the stator winding. ?
(A) 36 mWb
(B) 1.585 Hz, 3.17 per cent
(C) 1470 r/min, 1 Hz
(D) 4.6 per cent, 954 r/min
19 >>Explain the principle of action of a three-phase induction motor and the meaning of the term slip. How does slip vary with the load? A centre-zero d.c. galvanometer, suitably shunted,is connected in one lead of the rotor of a three-phase, six-pole, 50 Hz slip-ring induction motor and the pointer makes 85 complete oscillations per minute. What is the rotor speed? ?
(A) 971.7 r/min
(B) 1.585 Hz, 3.17 per cent
(C) 1470 r/min, 1 Hz
(D) 4.6 per cent, 954 r/min
20 >>Two similar three-phase star-connected generators are connected in parallel. Each machine has a synchronous reactance of 4.5 ?/ph and negligible resistance, andis excited to generate an e.m.f. of 1910 V/ph. The machines have a phase displacement of 30 electrical degrees relative to each other. Calculate: (a) the circulating current; (b) the terminal voltage/phase; (c) the active power supplied from one machine to the other. Sketch the phasor diagram for one phase. ?
